Light-duty liquid (“LDL”) detergents are commonly used in hand dishwashing liquids, shampoos, cleaners, and some laundry applications. The detergents are aqueous formulations that usually include an anionic surfactant, which is the principal suds producer, along with a secondary surfactant.
Because anionic surfactants tend to irritate skin and cause chapping, formulations that omit anionic surfactants (see, e.g., U.S. Pat. Appl. Publ. No. 2002/0169099) or include proteases, humectants, or other additives to counteract their undesirable effects (see, e.g., U.S. Pat. No. 4,451,385 and U.S. Pat. Appl. Publ. No. 2010/0197554) have been developed.
More often, however, anionic surfactants (typically, one or more of a linear alkylbenzene sulfonate, a fatty alcohol sulfate, or a fatty alcohol ether sulfate) are the primary surfactant components, and one challenge is to identify suitable secondary surfactants that can be used with the primary surfactant(s). Ideally, the secondary surfactant maintains or enhances sudsing performance and allows a reduced amount of anionic surfactant to be used. U.S. Pat. No. 7,884,064, for instance, teaches dish detergent formulations comprising sulfonated estolides (anionic surfactant) in combination with a fatty amine oxide, betaine, sulfobetaine, or alkanolamide as the secondary surfactant.
Occasionally, light-duty liquid detergents have been formulated to contain fatty esters or amides made by hydrolysis or transesterification of triglycerides, which are typically animal or vegetable fats. Consequently, the fatty portion of the acid or ester will typically have 6-22 carbons with a mixture of saturated and internally unsaturated chains. Depending on source, the fatty acid or ester often has a preponderance of C16 to C22 component. For instance, methanolysis of soybean oil provides the saturated methyl esters of palmitic (C16) and stearic (C18) acids and the unsaturated methyl esters of oleic (C18 mono-unsaturated), linoleic (C18 di-unsaturated), and α-linolenic (C18 tri-unsaturated) acids. These materials are generally less than completely satisfactory, however, because compounds having such large carbon chains can behave functionally as soil under some washing conditions.
Recent improvements in metathesis catalysts (see J. C. Mol, Green Chem. 4 (2002) 5) provide an opportunity to generate reduced chain length, monounsaturated feedstocks, which are valuable for making detergents and surfactants, from C16 to C22-rich natural oils such as soybean oil or palm oil. Soybean oil and palm oil can be more economical than, for example, coconut oil, which is a traditional starting material for making detergents. As Professor Mol explains, metathesis relies on conversion of olefins into new products by rupture and reformation of carbon-carbon double bonds mediated by transition metal carbene complexes. Self-metathesis of an unsaturated fatty ester can provide an equilibrium mixture of starting material, an internally unsaturated hydrocarbon, and an unsaturated diester. For instance, methyl oleate (methyl cis-9-octadecenoate) is partially converted to 9-octadecene and dimethyl 9-octadecene-1,18-dioate, with both products consisting predominantly of the trans-isomer. Metathesis effectively isomerizes the cis-double bond of methyl oleate to give an equilibrium mixture of cis- and trans-isomers in both the “unconverted” starting material and the metathesis products, with the trans-isomers predominating.
Cross-metathesis of unsaturated fatty esters with olefins generates new olefins and new unsaturated esters that can have reduced chain length and that may be difficult to make otherwise. For instance, cross-metathesis of methyl oleate and 3-hexene provides 3-dodecene and methyl 9-dodecenoate (see also U.S. Pat. No. 4,545,941). Terminal olefins are particularly desirable synthetic targets, and Elevance Renewable Sciences, Inc. recently described an improved way to prepare them by cross-metathesis of an internal olefin and an α-olefin in the presence of a ruthenium alkylidene catalyst (see U.S. Pat. Appl. Publ. No. 2010/0145086). A variety of cross-metathesis reactions involving an α-olefin and an unsaturated fatty ester (as the internal olefin source) are described. Thus, for example, reaction of soybean oil with propylene followed by hydrolysis gives, among other things, 1-decene, 2-undecenes, 9-decenoic acid, and 9-undecenoic acid. Despite the availability (from cross-metathesis of natural oils and olefins) of unsaturated fatty esters having reduced chain length and/or predominantly trans-configuration of the unsaturation, surfactants have generally not been made from these feedstocks.
We recently described new compositions made from feedstocks based on self-metathesis of natural oils or cross-metathesis of natural oils and olefins. In particular, we identified esteramines and ester quats, fatty amides, fatty amines and amidoamines, quaternized amines, betaines, sulfobetaines, alkoxylates, sulfonates, sulfo-estolides, and other compositions made by derivatizing the unique feedstocks (see copending PCT/US11/57596, PCT/US11/57597, PCT/US11/57595, PCT/US11/57602, PCT/US11/57605, and PCT/US11/57609, all filed Oct. 25, 2011. The feedstocks, which include C10-C17 monounsaturated acids and their ester derivatives, preferably have at least 1 mole % of trans-Δ9 unsaturation. Because performance of a particular surfactant or blend of surfactants in a light-duty liquid detergent is not easily inferred from surfactant structure, we performed extensive experimental investigations to identify subclasses of surfactants having desirable attributes for use in LDL detergents.
New surfactant classes are always of interest to formulators of light-duty liquid detergents. Surfactants based on renewable resources will continue to be in demand as alternatives to petroleum-based surfactants. Traditional natural sources of fatty acids and esters used for making surfactants generally have predominantly (or exclusively) cis-isomers and lack relatively short-chain (e.g., C10 or C12) unsaturated fatty portions. Metathesis chemistry provides an opportunity to generate precursors having shorter chains and mostly trans-isomers, which could impart improved performance when the precursors are converted to downstream compositions (e.g., in surfactants). Formulators will benefit from identification of particular subclasses of surfactants that derive from renewable sources and have desirable attributes for LDL detergents.